"Cool jazz" describes a style of music. "West Coast Jazz" was a marketing term coined by Dick Bock of the Pacific Jazz label, specifically for his artists Gerry Mullligan and Chet Baker. Since the music of many (but not all) of the "West Coast" artists can be considered "cool", the terms have become interchangeable in some peoples' minds. They shouldn't be, since a fair amount of cool jazz actually originated elsewhere, and a number of California musicians did not play "cool" jazz.